‘Aging’, or ‘senescence’ usually refers to the involutional changes that occur after an individual has reached full structural and functional maturity (Masoro, 1995). While it is clearly related to the passage of time, there is considerable variability among individuals in when the declines in function occur, the extent of the changes and the selective involvement of specific structures. The onset of impairment due to senescent changes, as well as the overall longevity of individuals, may vary by 50% or more. Senescent changes may involve the heart, joints, skin, brain or other organs in different individuals; and both the organ(s) involved and the degree of decline may be related to genetic and/or experiential factors. Adults between the ages of 65 and 85 years are classified as the ‘young-old’; those over age 85 as the ‘oldest-old’ (Suzman et al., 1992). As the elderly are the fastest growing segment of the population, there is much interest in the causes of senescence, and potential medical means of preventing or delaying their effects, many of which constitute the ‘degenerative diseases’ of the elderly.

Because of the variability in senescent decline, the concept of ‘normal’ aging has been controversial, and the term has been used in at least three different ways:

• The optimal level of function seen in individuals of a given age;

• The level of function seen in individuals of a given age in the absence of disease; and

• The mean level of function of all individuals of a given age.

Each of these concepts of ‘normal aging’, used in the appropriate context, is of value in recognizing and defining the usual expectation, specific disorders, and maximum potential of individuals with advancing age (Rowe & Kahn, 1987). The genetic endowment, the accidents of chance, and the wear and tear of lifetime experience assure that no one definition is necessarily the correct one. Regardless, all three definitions endorse the fact that some decline in function occurs over time; and the relationship between gradual decrements in performance with age, and more rapid and significant impairment of function with disease, is a key issue in understanding the neurologic basis of dementia.